A new technique for producing coatings on a wide variety of substrate surfaces by kinetic spray, or cold gas dynamic spray, was recently reported in two articles by T. H. Van Steenkiste et al. The first was entitled “Kinetic Spray Coatings”, published in Surface and Coatings Technology, vol. 111, pages 62-71, Jan. 10, 1999 and the second was entitled “Aluminum coatings via kinetic spray with relatively large powder particles”, published in Surface and Coatings Technology 154, pp. 237-252, 2002. The articles discuss producing continuous layer coatings having high adhesion, low oxide content and low thermal stress. The articles describe coatings being produced by entraining metal powders in an accelerated gas stream, through a converging-diverging de Laval type nozzle and projecting them against a target substrate. The particles are accelerated in the high velocity gas stream by the drag effect. The gas used can be any of a variety of gases including air, nitrogen or helium. It was found that the particles that formed the coating did not melt or thermally soften prior to impingement onto the substrate. It is theorized that the particles adhere to the substrate when their kinetic energy is converted to a sufficient level of thermal and mechanical deformation. Thus, it is believed that the particle velocity must exceed a critical velocity to permit it to adhere when it strikes the substrate. It was found that the deposition efficiency of a given particle mixture was increased as the main gas temperature was increased. Increasing the main gas temperature decreases its density and thus increases its velocity and increases its pressure. The velocity varies approximately as the square root of the main gas temperature. The actual mechanism of bonding of the particles to the substrate surface is not fully known at this time. The critical velocity is dependent on the material of the particle and of the substrate. Once an initial layer of particles has been formed on a substrate subsequent particles not only eliminate the voids between previous particles bound to the substrate by compaction, but also engage in particle to particle bonds. The bonding process is not due to melting of the particles in the main gas stream because the temperature of the particles is always below their melting temperature.
The above kinetic spray methods all relied on high pressure particle powder feeders. These powder feeders are very expensive and can cause erosion of the throat of the kinetic spray nozzle. In addition, high pressure systems are prone to clogging at the throat of the nozzle, which limits the main gas temperatures that can be used.
A recent improvement was disclosed in U.S. application Ser. No. 10/117,385, filed Apr. 5, 2002. In this improvement the particle powder is introduced through the side of the nozzle in the diverging section, which allows a low pressure powder feeder to be used. Low pressure powder feeders are very common, inexpensive and reliable. One problem encountered with both low pressure and high pressure nozzles is the inability of certain types of particles to achieve critical velocity even at higher main gas temperatures and pressures. Thus, it would be advantageous to design a supersonic nozzle allowing particles to achieve higher velocity with the same main gas temperature and pressure.